RESPIRATION. 



573 



until equilibrium is produced (Fig. 246). On the other hand, if the forces 

 which produce enlargement of the thorax cease to act, the elasticity of 

 the lungs, together with that of the cartilages of the ribs, is now sufficient 

 to produce a return of the thorax to its original volume. The lungs, 

 therefore, now decrease in volume, and as a consequence the air within 

 them tends to become compressed, and, as a result, a portion of the air 

 is expelled through the air-passages to the external atmosphere. The 

 expansion of the thorax constitutes inspiration ; the contraction of the 

 thorax constitutes expiration. 



As the tension of air in the lungs becomes decreased through 



inspiration, fresh air enters the lungs which is less charged with the 



' carbon dioxide than that previously present in the lungs, while it is also 



Fig. 246.— Diagrammatic Representation of the Relations between 

 the Lungs and the Thoracic Cavity, after Funke. (Beaunis.) 



The bell-jar, 1, represents the thorax ; the rubber membrane, 4, the diaphragm ; the membrane, 6, the 

 soft parts of an intercostal space ; 2, the trachea, terminating in two rnbber bulbs representing the lungs ; 

 3, a manometer for measuring the pressure within the bell-jar. 



In the figure to the left the atmospheric pressure within the bell-jar is the same as on the outside, and 

 the mercury in the manometer stands at the same level in both arms. If the rubber membrane, 4, is 

 drawn downward by the button, 5, the cavity of the bell-jar is increased and the atmospheric pressure 

 diminished, as shown by the manometer and the depressed space. 6. The negative pressure thus produced 

 leads to the entrance of air through the tube, 2, into the rubber bulbs, which consequently expand. The 

 action of the diaphragm in producing inspiration is precisely similar. 



richer in oxygen. By diffusion, from the inequality of these gaseous 

 tensions, we have oxygen brought to the lowermost strata of air in the 

 lungs and carbon dioxide diffusing from them, and when the air again 

 leaves the lungs in expiration it has been the means of introducing 

 oxygen into the lungs and removing carbon dioxide from them. 



The amount of air which ordinarily enters the lungs in inspiration 

 and is dispelled in expiration is spoken of as the tidal volume. By 

 forcible muscular contractions the capacity of the thorax may be both 

 increased and decreased beyond the dimensions present in gentle respi- 

 ration. The amount of air so drawn in by a forced inspiration is spoken 

 of as complemental air; that expelled from the lungs in violent expiration 

 is spoken of as supplemental air; while, even after forced expiration, a 



